Production method for a metal nanostructure using an ionic liquid

ABSTRACT

The present invention provides a method of forming metal nanostructures, and, more particularly, a method of uniformly forming various shapes of nanostructures, such as cubic or octahedral nanoparticles, nanowires and the like, using ionic liquid in a polyol reduction reaction in which metal salts are used as precursors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase of PCT International Patent Application No. PCT/KR2010/002127, filed Apr. 7, 2010, and Korean Patent Application No. 10-2009-0030599, filed Apr. 8, 2009, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a metal nanostructure, and, more particularly, to a method of uniformly forming various shapes of nanostructures, such as cubic or octahedral nanoparticles, nanowires and the like, using an ionic liquid in a polyol reduction reaction in which metal salts are used as precursors.

2. Description of the Related Art

Recently, research into the synthesis of metal nanoparticles has actively been made in order that they may be applied to flat panel displays, touch panels, solar cells, etc. Since these metal nanoparticles can be practically used to manufacture transparent electrodes, conductive ink and the like, it is required to develop a technology for producing the metal nanoparticles in large quantities. Further, since the shape of metal nanoparticles is an important factor affecting material properties, such as electroconductivity and the like, it is also required to develop a technology for freely controlling the shape of metal nanoparticles.

Recently, a technology for forming metal nanostructures, in which wire-shaped metal nanostructures are formed when a compound, such as polyvinyl pyrrolidone or the like, is used together with a polyol reductant, such as ethyleneglycol or the like, was reported in the paper (Chem. Mater. 14, 4736-4745). This technology is referred to as “a polyol reduction method”. This polyol reduction method is advantageous in that solution-phase metal nanostructures can be relatively easily formed. However, the metal nanostructures formed using the polyol reduction method are problematic in that various shapes are mixed although they chiefly have a wire shape, and in that it is difficult to produce the metal nanostructures such that their shape is reproducible according to reaction conditions.

Therefore, in the formation of metal nanostructures, it is required to develop a technology for uniformly and freely controlling the shapes of metal nanostructures, such as a wire shape, a cubic shape, an octahedral shape and the like.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method of freely and uniformly forming various shapes of metal nanostructures using an ionic liquid. According to the method, various shapes of metal nanostructures, such as a wire shape, a cubic shape, an octahedral shape and the like, are formed in a polyol reduction reaction in which metal salts are used as precursors.

Objects to be accomplished by the present invention are not limited to the above-mentioned object, and other objects can be clearly understood by those skilled in the art by the following descriptions.

In order to accomplish the above object, the present invention provides a method of forming various shapes of metal nanostructures, including the steps of: mixing an ionic liquid, a metal salt and a reducing solvent to form a mixture; and reacting the mixture.

In the method, the shape of the metal nanostructure may be determined by chemical bonding between cations and anions constituting the ionic liquid.

Further, in the method, various shapes of metal nanostructures including one-dimensional, two-dimensional and three-dimensional metal nanostructures may be formed by using various kinds of ionic liquids.

In the present invention, the shape of the metal nanostructure is varied by changing the anionic component of the ionic liquid in the polyol reduction reaction in which the metal salt is used as a precursor.

That is, various shapes of metal nanostructures may be formed by changing the kind of anion of the ionic liquid.

Metal salts are composed of a metal cation and an organic or inorganic anion. Examples of the metal salts of the present invention may include, but are limited to, AgNO₃, Ag(CH₃COO)₂, AgClO₄, Au(ClO₄)₃, PdCl₂, NaPdCl₄, PtCl₂, SnCl₄, HAuCl₄, FeCl₂, FeCl₃, Fe(CH₃COO)₂, CoCl₂, K₄Fe(CN)₆, K₄Co(CN)₆, K₄Mn(CN)₆, and K₂CO₃. The metal salt is converted into a corresponding metal nanostructure, such as a silver nanostructure, a gold nanostructure, a palladium nanostructure, a tin nanostructure, an iron nanostructure, a cobalt nanostructure or the like by a reduction reaction.

The reducing solvent is a polar solvent capable of dissolving the metal salt, and has two or more hydroxy groups in a molecule thereof, such as a diol, a polyol, a glycol or the like. Specific examples of the reducing solvent include ethyleneglycol, 1,2-propyleneglycol, 1,3-propyleneglycol, glycerin, glycerol, polyethyleneglycol, polypropyleneglycol, etc. The reducing solvent serves to produce a metal element by inducing the metal salt to be reduced.

The ionic liquid is a compound including an organic cation and an organic or inorganic anion, and is an imidazolium-based ionic liquid represented by Formula 1A below and/or a pyridinium-based ionic liquid represented by Formula 1B.

Here, R₁ and R₂ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C16, and may include a hetero atom; and X⁻ is an anion of the ionic liquid.

Here, R₃ and R₄ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C16, and may include a hetero atom; and X⁻ is an anion of the ionic liquid.

Specific examples of the cation of the imidazolium-based ionic liquid represented by Formula 1A above may include 1,3-dimethylimidazolium, 1,3-diethylimidazolium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-octyl-3-methylimidazolium, 1-decyl-3-methylimidazolium, 1-dodecyl-3-methylimidazolium, 1-tetradecyl-3-methylimidazolium, and the like. Specific examples of the cation of the pyridinium-based ionic liquid represented by Formula 1B above may include 1-methylpyridinium, 1-ethylpyridinium, 1-butylpyridinium, 1-ethyl-3-methylpyridinium, 1-butyl-3-methylpyridinium, 1-hexyl-3-methylpyridinium, 1-butyl-3,4-dimethylpyridinium, and the like.

Further, the ionic liquid of the present invention may include a polymeric ionic liquid as well as the monomolecular ionic liquid represented by Formula 1A or 1B. Examples of the polymeric ionic liquid may include, but are not limited to, poly(1-vinyl-3-alkylimidazolium), poly(1-vinyl-pyridinium), poly(1-vinyl-alkylpyridinium), poly(1-allyl-3-alkylimidazolium), and poly(1-(meth)acryloyloxy-3-alkylimidazolium).

The monomolecular or polymeric ionic liquid includes an organic or inorganic anion. Example of the organic or inorganic anion may include, but are not limited to, Br⁻, Cl⁻, I⁻, BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻, NO₃ ⁻, AlCl₄ ⁻, Al₂Cl₇ ⁻, AsF₆ ⁻, SbF₆ ⁻, CH₃COO⁻, CF₃COO⁻, CH₃SO₃ ⁻, C₂H₅SO₃ ⁻, CH₃SO₄ ⁻, C₂H₅SO₄ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, (CF₃SO₂)₃C⁻, (CF₃CF₂SO₂)₂N⁻, C₄F₉SO₃ ⁻, C₃F₇COO⁻, (CF₃SO₂)(CF₃CO)N⁻.

The monomolecular or polymeric ionic liquid can exhibit various physical and chemical properties depending on the combination of cation and anion which is used, and, preferably, may be a monomolecular or polymeric ionic liquid compatible with a metal salt and a reducing solvent. Since the ionic liquid serves to help a metal element to grow in one, two or three dimensions thanks to the chemical interaction between the ionic liquid and the metal ion or the metal element when a metal salt is converted into the metal element by polyol reduction, uniformly-shaped metal nanoparticles are finally formed.

In particular, the anionic component of the ionic liquid controls the shape of the finally formed metal nanoparticles. For example, when the ionic liquid containing a sulfur compound anion such as an alkyl sulfate anion (RSO₄ ⁻) or alkyl sulfonate anion (RSO₃ ⁻) is used, a one-dimensional metal nanostructure, such as a nanowire, is formed; and when the ionic liquid containing a halide anion is used, a three-dimensional metal nanostructure is formed, and, particularly, when the ionic liquid containing a chlorine anion (Cl⁻) is used, a cubic metal nanostructure is formed, and when the ionic liquid containing a bromine anion (Br⁻) is used, an octahedral metal nanostructure is formed. That is, various shapes of metal nanostructures may be selectively formed by changing the kind of the anionic component of the ionic liquid. The interaction between the nanoparticles and the ionic liquid in the early stage of reaction changes the growth direction of metal nanoparticles, thereby varying the shape of the metal nanostructure. Therefore, in this stage, the anion of the ionic liquid plays an important role in shaping the metal nanostructure. That is, in the early stage of the reaction, first, a metal salt is reacted with a reducing solvent to form metal nanoparticles, and then the formed metal nanostructures interact with the anion of the ionic liquid to grow in a predetermined direction, thereby forming various shapes of metal nanostructures.

A method of forming a metal nanowire which is a typical example of the nanostructures of the present invention is conducted as follows. First, a metal salt, a reducing solvent and an ionic liquid containing a sulfur compound anion are mixed at a predetermined mixing ratio and then stirred at room temperature for a predetermined time to form a uniform mixture, and then the mixture is reacted at 150˜200° C. to form a metal nanowire. The nanowire formed in this way has an average diameter of 0.01˜0.1 μm and an average length of 5˜100 μm. In the method, in order to form the nanowire, it is required to properly adjust the mixing ratio of the ionic liquid, metal salt and reducing solvent. In the mixing ratio thereof, the metal salt may have a concentration of 0.01˜1 M based on the reducing solvent, and the ionic liquid (its repetitive unit when it is a polymeric ionic liquid) may have a concentration of 0.001˜1 M based on the reducing solvent. When the concentration of the metal salt is below 0.01 M, only a small amount of the metal nanowire is formed because the concentration of the metal salt is excessively low, and when the concentration of the ionic liquid is below 0.001 M, the metal nanowire cannot be easily formed because the amount of the ionic liquid is excessively small. In contrast, when the concentration of the metal salt is above 1 M, the formed metal nanowires adhere to each other and the sizes thereof are increased, and when the concentration of the ionic liquid is above 1 M, it is difficult to synthesize the metal nanowire because the viscosity of the mixed solution is excessively high.

When ionic liquids including different kinds of anions are used in the above method, cubic metal nanostructures or octahedral metal nanostructures can be uniformly and stably synthesized.

In the method of forming a metal nanostructure according to the present invention, in order to effectively control the shape and size of the metal nanostructure, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below may be used as additives:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and may include a hetero atom; and r is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and may include a hetero atom; and Y⁻ is an organic or inorganic anion.

In this case, the nitrogen compound or the sulfur compound may have a content of 0.1˜100 parts by weight based on 100 parts by weight of the metal salt. When the content of the nitrogen compound or the sulfur compound is below 0.1 parts by weight, it is slightly effective to control the shape and size of the metal nanostructure; also, the harmful side effect of deformation of the shape of the nanostructure happens when the content thereof is above 100 parts by weight.

Examples of the nitrogen compound represented by Formula 2A above include tetrabutyl ammonium chloride, cetyltrimethyl ammonium bromide, tetrabutyl phosphonium chloride, and the like. Examples of the sulfur compound represented by Formula 2B above include sodium dodecyl sulfate, dodecyl benzene sulfonate, polystyrene sulfonate, poly(sodium-4-styrene sulfonate), and the like.

According to the present invention, various shapes of metal nanostructures can be formed by mixing and reacting an ionic liquid, a metal salt and a reducing solvent.

Further, in a polyol reduction reaction in which a metal salt is used as a precursor, when ionic liquids having different kinds of anions are selectively used, different shapes of metal nanostructures can be selectively reproducibly formed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIGS. 1 to 3 are photographs showing metal nanostructures formed using the method according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention will be described in more detail by the following Examples. Here, the following Examples are set forth to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

50 mL of a solution in which silver nitrate (AgNO₃) is dissolved in ethyleneglycol to a concentration of 0.1 M was mixed with 50 mL of a solution in which 1-butyl-3-methylimidazolium methyl sulfate is dissolved in ethyleneglycol to a concentration of 0.15 M in a round-bottom flask to form a mixed solution. Subsequently, the mixed solution was stirred and reacted at 160° C. for 60 minutes, and was then cooled to room temperature. Subsequently, the cooled mixed solution was filtered with a filter having a pore size of 1 μm, and was then observed with an electron scanning microscope. As a result, it was found that metal nanowires were formed, as shown in FIG. 1. It was observed that the metal nanowires had a diameter of about 220 nm and a length of about 7 μm.

Example 2

10 mL of a solution in which silver nitrate (AgNO₃) is dissolved in 1,3-propyleneglycol to a concentration of 0.2 M was mixed with 10 mL of a solution in which 1-ethyl-3-methylimidazolium methyl sulfate is dissolved in 1,3-propyleneglycol to a concentration of 0.3 M in a round-bottom flask to form a mixed solution. Subsequently, the mixed solution was stirred and reacted at 100° C. for about 30 minutes, and was then cooled to room temperature. Subsequently, the cooled mixed solution was filtered with a filter having a pore size of 1 μm, and was then observed with an electron scanning microscope. As a result, it was found that metal nanowires having a diameter of about 180 nm and a length of about 10 μm were formed.

Example 3

10 mL of a solution in which silver nitrate (AgNO₃) is dissolved in 1,2-propyleneglycol to a concentration of 0.2 M was mixed with 10 mL of a solution in which 1-ethyl-3-methylimidazolium methyl sulfate is dissolved in 1,3-propyleneglycol to a concentration of 0.3 M in a round-bottom flask to form a first mixed solution, and then sodium dodecyl sulfate was added to the first mixed solution in an amount of 1% of the silver nitrate (AgNO₃) to form a second mixed solution. Subsequently, the second mixed solution was stirred and reacted at 100° C. for about 30 minutes, and was then cooled to room temperature. Subsequently, the cooled second mixed solution was filtered with a filter having a pore size of 1 μm, and was then observed with an electron scanning microscope. As a result, it was found that metal nanowires having a diameter of about 80 nm and a length of about 10 μm were formed.

Example 4

Metal nanostructures were formed in the same manner as in Example 1, except that 1-ethyl-3-methylpyridinium methyl sulfate was used as an ionic liquid. As in Example 1, the cooled mixed solution was filtered with a filter having a pore size of 1 μm, and was then observed with an electron scanning microscope. As a result, it was found that metal nanowires were formed. It was observed that the metal nanowires had a diameter of about 320 nm and a length of about 5 μm.

Example 5

Metal nanostructures were formed in the same manner as in Example 1, except that 1-butyl-3-methylimidazolium chloride was used as an ionic liquid. The cooled mixed solution was filtered with a teflon filter having a pore size of 0.2 μm, and was then observed with an electron scanning microscope. As a result, it was found that cubic silver nanoparticles having a particle size of about 400 nm, as shown FIG. 2.

Example 6

Metal structures were formed in the same manner as in Example 1, except that 1-butyl-3-methylimidazolium bromide was used as an ionic liquid. The cooled mixed solution was filtered with a filter having a pore size of 1 μm, and was then observed with an electron scanning microscope. As a result, it was found that octahedral silver particles having a particle size of about 5 μm, as shown FIG. 3.

As described above, the metal nanostructures formed using the method of the present invention can be used in various industrial fields, such as flat panel displays, touch panels, solar cells, etc. 

1. A method of forming various shapes of metal nanostructures, comprising the steps of: mixing an ionic liquid, a metal salt and a reducing solvent to form a mixture; and reacting the mixture.
 2. The method according to claim 1, wherein the shape of the metal nanostructure is determined by chemical bonding between a cation and an anion constituting the ionic liquid.
 3. The method according to claim 2, wherein the ionic liquid containing a sulfur compound anion such as an alkyl sulfate anion (RSO₄ ⁻) or alkyl sulfonate anion (RSO₃ ⁻) is used to form a one-dimensional metal nanostructure such as a nanowire, the ionic liquid containing a halide anion is used to form a three-dimensional metal nanostructure, the ionic liquid containing a chlorine anion (Cl⁻) is used to form a cubic metal nanostructure, and the ionic liquid containing a bromine anion (Br) is used to form an octahedral metal nanostructure.
 4. The method according to claim 3, wherein the ionic liquid is a compound including an organic cation and an organic or inorganic anion, and is a monomolecular compound or a polymeric compound.
 5. The method according to claim 4, wherein the ionic liquid includes an imidazolium-based ionic liquid represented by Formula 1A below and a pyridinium-based ionic liquid represented by Formula 1B below:

wherein R₁ and R₂ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C16, and includes a hetero atom; and X⁻ is an anion of the ionic liquid,

wherein R₃ and R₄ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C16, and includes a hetero atom; and X⁻ is an anion of the ionic liquid.
 6. The method according to claim 4, wherein the anion of the ionic liquid is any one selected from Br⁻, Cl⁻, I⁻, BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻, NO₃ ⁻, AlCl₄ ⁻, Al₂Cl₇ ⁻, AsF₆ ⁻, SbF₆ ⁻, CH₃COO⁻, CF₃COO⁻, CH₃SO₃ ⁻, C₂H₅SO₃ ⁻, CH₃SO₄ ⁻, C₂H₅SO₄ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, (CF₃SO₂)₃C⁻, (CF₃CF₂SO₂)₂N⁻, C₄F₉SO₃ ⁻, C₃F₇COO⁻, and (CF₃SO₂)(CF₃CO)N⁻.
 7. The method according to claim 1, wherein the metal salt is composed of a metal cation and an organic or inorganic anion, and is any one selected from AgNO₃, Ag(CH₃COO)₂, AgClO₄, Au(ClO₄)₃, PdCl₂, NaPdCl₄, PtCl₂, SnCl₄, HAuCl₄, FeCl₂, FeCl₃, Fe(CH₃COO)₂, CoCl₂, K₄Fe(CN)₆, K₄Co(CN)₆, K₄Mn(CN)₆, K₂CO₃.
 8. The method according to claim 1, wherein the reducing solvent is a solvent having two or more hydroxy groups in a molecule thereof, such as a diol, a polyol, a glycol or the like, and is any one selected from ethyleneglycol, 1,2-propyleneglycol, 1,3-propyleneglycol, glycerin, glycerol, polyethyleneglycol, and polypropyleneglycol.
 9. The method according to claim 1, wherein, in a mixing ratio of the ionic liquid, metal salt and reducing solvent, the metal salt has a concentration of 0.01˜1 M based on the reducing solvent, and the ionic liquid (its repetitive unit when it is a polymeric ionic liquid) has a concentration of 0.001˜1 M based on the reducing solvent.
 10. The method according to claim 1, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion.
 11. The method according to claim 10, wherein the nitrogen compound or the sulfur compound has a content of 0.1˜100 parts by weight based on 100 parts by weight of the metal salt.
 12. A metal nanostructure formed using the method of claim
 1. 13. The method according to claim 5, wherein the anion of the ionic liquid is any one selected from Br⁻, Cl⁻, I⁻, BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻, NO₃ ⁻, AlCl₄ ⁻, Al₂Cl₇ ⁻, AsF₆ ⁻, SbF₆ ⁻, CH₃COO⁻, CF₃COO⁻, CH₃SO₃ ⁻, C₂H₅SO₃ ⁻, CH₃SO₄ ⁻, C₂H₅SO₄ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, (CF₃SO₂)₃C⁻, (CF₃CF₂SO₂)₂N⁻, C₄F₉SO₃ ⁻, C₃F₇COO⁻, and (CF₃SO₂)(CF₃CO)N⁻.
 14. The method according to claim 3, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion.
 15. The method according to claim 4, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion.
 16. The method according to claim 5, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion.
 17. The method according to claim 6, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion.
 18. The method according to claim 7, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion.
 19. The method according to claim 8, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion.
 20. The method according to claim 13, wherein, in addition to the ionic liquid, metal salt and reducing solvent, a nitrogen compound represented by Formula 2A below or a sulfur compound represented by Formula 2B below is used as an additive:

wherein R₅, R₆, R₇ and R₈ are identical to or different from each other, are each independently selected from hydrogen and a hydrocarbon group of C1-C20, and includes a hetero atom; and Y⁻ is an organic or inorganic anion,

wherein R is a monomolecular or polymeric hydrocarbon group, and includes a hetero atom; and Y⁻ is an organic or inorganic anion. 